In the manufacture of disc drives, the design effort in regard to head positioning devices has previously been directed to minimizing the seek time as the head is moved from one data track to another. This is normally accomplished by accelerating the head at a constant rate up to the midpoint (in time) of the seek, and then decelerating at the same rate so that the head velocity falls to zero at the destination track. The resulting acceleration-deceleration profile is known as a time-optimal or minimum access profile. In practice, this method, though effective, requires expensive electronics, precision in manufacture, and the provision of a position sensor for assuring accurate capture of the destination track.
There are many applications for disc drives in which the cost of the unit is more important than time-optimal performance. The approach described in my copending application Ser. No. 735,851, filed Oct. 26, 1976, entitled METHOD AND MEANS FOR CAPTURING MAGNETIC TRACKS; Ser. No. 735,852, filed Oct. 26, 1976, entitled METHOD AND MEANS FOR TRACKING MAGNETIC TRACKS: and Ser. No. 735,716, filed Oct. 26, 1976, entitled METHOD AND MEANS FOR INCREASING THE STIFFNESS OF LIMITED FREQUENCY SERVO SYSTEMS, permits the cost of a disc drive to be substantially reduced as compared to conventional units while retaining most of their significant performance characteristics.
As explained in the aforementioned copending application Ser. No. 735,851 entitled METHOD AND MEANS FOR CAPTURING MAGNETIC TRACKS, the head positioning system used in my low-cost disc drive avoids the use of a position sensor by making a blind seek, capturing the second track reached by the head after its velocity drops below a predetermined capture velocity, and making another seek if the captured track is not the desired one. Very short seeks (e.g. 16 tracks or less) are more advantageously handled by digitally counting track crossings, because the low head velocities encountered on such very short seeks allow accurate digital counting; and therefore such very short seeks are not involved in the present invention.